Alkaline electrochemical cells typically contain an alkaline electrolyte, such as potassium hydroxide, a cathode comprised of a metal oxide, such as manganese dioxide, and a zinc anode. These cells provide excellent performance and are used throughout the world in many consumer applications.
A detrimental characteristic of these cells is the formation of hydrogen gas. This gas can be formed by undesirable chemical reactions which occur at the surface of the anode current collector. As the quantity of gas increases, the internal pressure in the cell also increases. If this pressure increase is not relieved, the cells can eventually leak.
A widely used solution to the detrimental reactions which occur at the surface of the collector is the addition of mercury to the anode. Mercury in the anode plates onto the surface of the collector thereby inhibiting the formation of gas and thus enhancing the performance of the electrochemical cell. Unfortunately, as is well known, mercury may add to environmental pollution when cells are eventually discarded.
To address this environmental concern, the amount of mercury used in the cell has been lowered by adding zinc corrosion inhibitors to the cell. Examples of these inhibitors include lead, indium, cadmium, thallium, gold, silver, tin, gallium and compounds that incorporate these elements. These inhibitors have been alloyed with the zinc, deposited on the zinc, included in the electrolyte and deposited on the collector. Organic inhibitors, such as polyethylene glycol, have also been tried. These methods have led to the commercialization of low mercury content batteries, called "Ultra-low Mercury," that have about 250 parts of mercury per million parts based on total battery weight.
Unfortunately, these known inhibitors do not reliably permit the total removal of mercury from the cell. For example, indium is an effective inhibitor at certain levels of mercury, but surprisingly, at lower levels of mercury, indium is not as effective. The organic inhibitors are effective for cells that are undischarged and stored, but they do not inhibit gassing for cells that are partially discharged and then stored.
A deficiency found in some of the mercury free cells known to the inventor is the occurrence of an unexpected depression in the cell's voltage during discharge of the cell. These deviations from the expected voltage curve are usually temporary. Typically, the cell's voltage recovers and the cell proceeds to provide a reasonable amount of electrical service to the customer. This type of temporary depression and then recovery in voltage is referred to as a "dip". However, in some cases, the voltage does not recover from the early dip and the useful life of the cell is terminated prematurely thereby resulting in a "dud." Both of these problems, dips and duds, are undesirable.
In U.S. Pat. No. 4,942,101, figures five, six, seven and eight illustrate alkaline cells whose voltages "dip" during discharge and then recover. This patent is directed to an alkaline electrochemical cell that (1) is free of mercury, (2) incorporates specified quantities of selected organic stabilization compounds in the zinc powder, (3) utilizes an anode current collector that is constituted, at least superfically, by a substance selected from: pure zinc, pure cadmium, indium, and gallium, and (4) the surface of the current collector has been modified without melting of the metal so as to increase its developed surface area. While the invention described in this patent may solve the problem as described therein, the use in a commercially viable mass production process of a current collector that has been physically modified is potentially cumbersome and therefore expensive.
In view of these disadvantages, there is a need for an alkaline electrochemical cell that has a low mercury content or is free of mercury, does not exhibit either a temporary depression (i.e. dip) or a permanent drop (i.e. dud) in the voltage of the cell, does not exhibit excessive bulging and can be economically mass produced in a commercially viable process.